As the use of virtual environments (e.g., electronic marketplaces) increases, operators of those virtual environments are becoming increasingly interested in the effect of even minor changes to those virtual environments. For example, as content in an environment is altered, operators often need to be made aware of the effect that the alteration has had on visitors to the virtual environment. Effects may be found via the use of experiments. However, frequent visitors tend to enter into experiments earlier and are, in turn, followed for a longer period of time with current approaches. As a consequence, results of current analyses can be seen as the weighted average of long-term cumulative effects of frequent visitors and the short-term cumulative treatment of infrequent visitors. In other words, there may exist some self-selection bias in current approaches, which impairs the interpretability of the conclusions.